1. Field of the Invention
This invention relates to low-power high-speed motors which are operated at speeds of up to 20,000 rpm, and more particularly to a bearing that operates with the lowest possible friction and without play for such motors.
2. Discussion of Related Art
Motors of the type to which the present invention applies are typically employed as the drive in disk storage devices, as well as in the drives for scanners, copiers and similar small devices.
All such motor designs have in common the fact that a rotor dome, which is driven to rotate, is arranged on a stationary base flange and is supported on the base flange with appropriate axial and radial bearings. It is a known drive principle to arrange a stator package on the base flange so that it works together with a magnet arranged over an air gap with a driven rotor on the inside, that is, the so-called rotor dome. Thus, rotation of the rotor dome is induced by the magnetic return due to an appropriately applied rotational field in the stator package.
It is customary with such low-power motors to equip the rotor, that is, the rotor dome, which is rotating at a high rpm, with radial and axial bearings. The bearings should operate with the lowest possible friction. A typical design for such a bearing arrangement consists of using a grooved ball bearing which acts both as a radial bearing and as an axial bearing. Such ball bearings have the disadvantage that they must be lubricated and may sometimes cause mechanical vibrations which are unwanted and shorten the useful life of the motor. Furthermore, this vibration can have a negative effect on the object arranged on the rotor dome, for example, a data storage device (hard drive), so that accuracy in reading and writing is thereby impaired.
It is also known that the radial bearing component can be separated from the axial bearing component by arranging two different bearings, each of which assumes one bearing function. It is known that a so-called needle bearing or roller bearing, which takes up the radial bearing component, may be used as the radial bearing, and it is also known that a ball bearing or a fluid bearing may be used as the axial bearing.
Additionally, the bearing combination of two air cushion bearings or two fluid bearings is known. In this case it is known that an air cushion bearing is provided to take up the radial bearing components as well as a second air cushion bearing which is provided to take up the axial bearing components.
The arrangement of an air cushion bearing to take up axial bearing components is relatively difficult and expensive because the required tolerances in this axial bearing clearance must be kept very narrow, which greatly increases the manufacturing costs of such a bearing.
U.S. Pat. No. 5,289,067 discloses a small motor having radial and axial bearing components. A rotationally driven rotor dome is mounted on a shaft of a stationary base flange by the bearing components. The radial bearing is designed as a hydrodynamic bearing and the axial bearing is designed as a passive magnetic bearing in the form of a ring-shaped permanent magnet in which a magnetically active metal disk is arranged centrally and has a rotationally fixed connection to the rotor dome.
A primary purpose of this invention is to improve upon a low-power motor having a bearing arrangement of the type defined above so that in the case of the presence of a radial bearing of any type, the cost of production of the respective and separate axial bearing is significantly reduced, while at the same time achieving substantially improved bearing properties.
An important feature of this invention is that a magnetic axial bearing is designed as an active magnetic bearing and has two coils which serve to produce two independent magnetic fields which act on the metal disk arranged between the two coils, with the metal disk being attached in a rotationally fixed but detachable manner to the rotor dome by the magnetic force of a permanent magnet.
An important advantage of the invention is that now a magnetic bearing designed as an active magnetic bearing is used as the axial bearing.
In a first embodiment of the present invention, the active magnetic bearing operates on one side against a magnetic bias, while in a second embodiment, the active magnetic bearing operates on two sides against a magnetic bias applied from both sides.
Another significant feature of this invention is that the axial magnetic bearing is arranged on one side, that is, on the one side of the rotor dome, and not on both sides. This yields the important advantage that the bearing is much less expensive because, thanks to this arrangement, the axial magnetic bearing is capable of accommodating both axial forces acting in one direction is well as in the other direction, although there is only a single bearing. This also yields the additional advantage that dismantling is simplified because the rotor dome can be removed simply by lifting it up, so the axial magnetic bearing arranged inside the rotor dome is separated. Thus, it is not necessary to dismantle a second bearing (not included in this configuration) as might be the case in the known related art.
It is thus important that the active magnetic bearing can be arranged either on the top inside of the rotor dome or in a different embodiment on the lower side of the rotor dome. In both embodiments, it is important that only a single axial magnetic bearing is present, because it operates against biases on both sides so that it is capable of taking up the axial bearing forces acting in both directions. This greatly reduces the cost of manufacturing such a low-power motor, and furthermore, the rotor dome is stabilized because it is protected against tilting by the axial magnetic bearing. There are no mechanical vibrations such as those known to occur with mechanical bearings, and thus it runs more smoothly and quietly. This also results in an improved useful life for the motor.
In a preferred embodiment of the present invention, the axial magnetic bearing consists essentially of a coil with a magnetic return ring which is thermally separated from the radial air cushion bearing. This coil acts in the axial direction on an armature which is consequently attracted by this coil to varying extents in the axial direction. After the armature is fixedly connected to the rotor dome, the rotor dome is thus more or less adjusted in the axial direction due to appropriate energization of the coil in the axial direction.
The axial bias of the rotor dome against which the coil, which receives an electric current, operates is achieved in the first embodiment by means of two oppositely polarized permanent magnets which, for example, repel one another with their two identical poles and thus exert a repulsion effect on the rotor dome which is in turn counteracted by the coil receiving the electric current. In this way, the axial bearing play of the rotor dome can be adjusted with an extremely high precision through the electric current acting on the coil.
To set a certain dimension, it is preferable to use a sensor that monitors the axial bearing play and is connected to the power supply of the coil through an assigned control system, thus guaranteeing automatic control of the bearing play at a constant level. This bearing play is, of course, designed to be adjustable.
Various embodiments of position sensors may be used here. In a first embodiment, there is a sensor that acts by capacitance or inductance, measuring the distance in the bearing gap from a stationary face to the opposite rotating face and detecting it accordingly. In another embodiment, the sensor may be designed as an optical sensor, and the adjustment of the axial bearing play may take place by means of an optical sensor.
In a third embodiment, an inductive sensor may be provided in such a manner that a stationary induction coil in which a suitable induction voltage is induced is arranged opposite the rotating magnet arranged in the rotor dome. Depending on the size of the axial air gap between these two parts, an induction voltage of various levels is induced in the induction coil in direct proportion to the axial bearing play. Likewise, the bearing play can also be regulated with a high precision in this way.
In another embodiment of the present invention, instead of a permanent magnetic bias of the rotor:dome in the axial direction, an active bias can be produced by two oppositely directed coils forming an air gap between them by the rotation of a disk in fixed rotational connection to the rotor dome. Depending on the current acting on the upper and lower coils, the metal disk which is fixedly connected to the rotor dome is moved upward or downward axially, thus also adjusting the magnetic bearing accordingly.
Whereas in the first embodiment a current-carrying coil and a paired arrangement of permanent magnets was used, in the second embodiment two current-carrying coils are used, forming an. air gap between them where a metallic and magnetically active disk rotates, the disk being attached to the rotor dome in a rotationally fixed manner. Depending on the current flowing in the upper and lower coils, the disk connected to the rotor thus moves upward or downward axially, again with precise control of this axial adjustment play being achieved due to the above-mentioned position sensor and the above-mentioned control.
In a third embodiment of the present invention, an air cushion bearing is again combined with an axial magnetic bearing, but in this case the air cushion bearing does not function as a purely radial bearing but instead is capable of taking up both radial and axial bearing components. It is preferable here if this air cushion bearing is designed as a toroidal bearing which, by definition, takes up both axial and radial bearing components.
In the design of the active magnetic bearing as an axial bearing, the two above-mentioned embodiments are again possible. This means that it is preferable according to this invention if a cylindrical air cushion bearing or a toroidal bearing is used as the radial bearing, while a magnetic bearing is always used as the axial bearing.
In another embodiment, the bearing pair combining the above-mentioned radial and essentially cylindrical air cushion bearing with an axial air cushion bearing which is in turn combined with the above-mentioned axial magnetic bearing is also used. This design uses an air cushion bearing arrangement consisting of a radial air cushion bearing in combination with an axial magnetic bearing. This also yields some important advantages because more accurate axial positioning of the rotor dome is possible due to a well-thought-out arrangement of an additional axial air cushion bearing. This additional axial air cushion bearing then acts as an additional carrier which stabilizes the tilting vibrations and thus also leads to a further improvement in the smooth running of the rotor.